At present, as a substrate for a flexible device such as an electronic paper, a plastic substrate made of e.g. a polycarbonate or a polyethylene terephthalate has been studied, but there is a problem that such a substrate is likely to be slightly expanded/shrunk when heated, and accordingly it is acutely desired to improve the heat resistance. On the other hand, in order to reduce a thermal stress applied to a plastic substrate, it has been studied to lower the temperature in a process for producing an organic transistor. One of process steps requiring the highest temperature in the production of an organic transistor, is a step for forming a gate insulating film, in which the temperature is desired to be lowered.
As a process for forming a gate insulating film at a lower temperature, e.g. a process for anodic oxidation of the surface of gate electrode (Patent Document 1) or a chemical vapor deposition process (Patent Document 2) has been proposed, but such a process is cumbersome, and therefore it is desired to employ a material which can readily be formed into a film by coating, such as spin coating or printing method.
On the other hand, as an example of preparing a gate insulating film by coating, firstly, an example may be mentioned, in which poly-4(vinyl phenol) and poly(melamine-formaldehyde) are cured at 200° C. (Non-Patent Document 1). However, in this example, the baking temperature is high at a level of 200° C., and at such a temperature, the influence over the heat-expansion or shrinkage of a plastic substrate will be remarkable, whereby it becomes difficult to prepare an electronic paper having fine pixels. Further, another example may be mentioned, in which a polyimide precursor curable at a low-temperature is baked at 180° C. (Non-Patent Document 2). However, this example mentions a polyimide simply and discloses no specific structures of the polyimide. Further, a leak current density is disclosed to be at least 1×10−9 A/cm2 at 2 MV/cm, and the insulation properties are still insufficient.
Further, Patent Document 3 discloses a polyimide obtained by imidation of a polyimide precursor made of cyclobutene tetracarboxylic acid dianhydride and a specific diamine having a hexafluoropropylidene group in its molecule, discloses that it is possible to obtain a polyimide film having excellent transparency not only in the visible portion but also in the ultraviolet portion even in the baking at a high temperature of from 270 to 350° C., and discloses that such a polyimide film is suitably used for e.g. a protective film for a liquid crystal display device or a semiconductor device, an insulating film or an optical waveguide material for optical communication. Further, in Examples thereof, a polyimide precursor obtained from 2,2′-bis(3-methyl-4-aminophenyl)hexafluoropropane and cyclobutane tetracarboxylic acid dianhydride, is baked at 300° C. to form a polyimide film. However, Patent Document 3 is silent about formation of a polyimide film by a low-temperature baking suitable for a gate insulating film, and therefore there is no disclosure at all which may suggest a structure of a polyamide precursor or its relation with e.g. film-forming conditions, for obtaining a polyimide film having a good film quality by a low-temperature baking.
Patent Document 1: JP-A-2003-258260
Patent Document 2: JP-A-2004-72029
Patent Document 3: WO2000/22049
Non-Patent Document 1: “Journal of Applied Physics (J. Appl. Phys.)” Vol. 93, No. 5, 1 Mar. 2003, p. 2997-2981
Non-Patent Document 2: “Applied Physics Letters (Appl. Phys. Lett.)” Vol. 84, No. 19, 10 May 2004, p. 3789-3791